Imaging of low-light-level environments is an important capability for enabling military and law enforcement surveillance, aviation and automotive navigation, and for numerous industrial and consumer manufacturing and production processes, among other applications. For example, the conventional low-light-level imaging systems known as so-called night vision scopes are routinely employed as a principal means for enabling night time mobility and navigation by military personnel wearing helmet-mounted scopes while traveling on foot or navigating in a vehicle, e.g., a jeep, truck, helicopter or jet.
Historically, low-light-level imaging systems, including conventional night vision scopes, have been based on use of an electro-optic image sensor that provides a gain mechanism for amplifying ambient input light to produce an output image signal level that is adequate for visual display, e.g., either by a direct view display or, with the addition of an electronic imaging system, a remote view display. For example, in a typical electro-optic system such as a so-called intensifier tube imager, a lens is used to focus ambient light, such as moonlight or starlight reflected off of a scene, onto a photocathode in a vacuum tube, the photocathode being sensitive to, e.g., light from the yellow through near-infrared portion of the electromagnetic spectrum. Under application of a high voltage between the photocathode and a micro-channel plate also located in the vacuum tube, an input ambient photon incident on the photocathode causes emission of a single electron from the photocathode and acceleration of the electron toward the micro-channel plate. Upon striking the micro-channel plate, the single electron creates a cascade of many electrons, which together are accelerated toward a phosphor screen by a second applied voltage. The kinetic energy of the electrons striking the phosphor causes the phosphor to glow. This electron cascade mechanism results in amplification of the input ambient light, typically by about four orders of magnitude, to produce a visible image on the phosphor screen. The phosphor image exists only momentarily in the glow of the phosphor screen, and does not exist in a storable or readable form.
To produce a real-time electronic video image, which typically is set at a frame rate of about 30 frames/second, based on the optical images formed by an intensifier tube on a phosphor screen, it is common practice to optically couple the phosphor screen to a conventional charge-coupled-device (CCD) electronic imaging camera. Optical coupling is typically achieved using an intermediate coupling lens or an optical fiber taper bonded to the CCD and either bonded or integrally-connected to the intensifier tube phosphor screen. The resulting low-light video camera, or so-called intensified-CCD camera, relies entirely on the cascade gain mechanism of the intensifier tube to provide a phosphor image that is adequately amplified to be sensed by the conventional CCD imager.
Intensified-CCD cameras like the one described above, while capable of producing a real time low-light-level video sequence, have historically been severely restricted with regard to other performance criteria. In particular, the intra-scene dynamic range of an image produced by an intensified-CCD imaging system is severely limited by the image intensifier tube. Furthermore, image resolution is severely degraded at low light levels due to electronic noise associated with the intensifier tube cascading gain mechanism. This electronic noise adds background image intensity noise and can even "swamp" low intensity images, resulting in an output image that is a poor rendition of the imaged scene. Intensifier tube imagers not including a CCD electronic camera are of course also subject to the resolution and intra-scene dynamic range limitations imposed by the intensifier tube gain mechanism.
These limitations are exacerbated in imaging low-light-level scenes because the same scene may contain very low brightness areas as well as dramatic intra-scene intensity fluctuations due, e.g., to man-made light. But because intensifier tube imagers and intensified-CCD imaging systems intrinsically rely on the vacuum tube cascading gain mechanism for production of a viable phosphor image, the dynamic range limitation imposed by the gain mechanism must be accepted, resulting in either loss of darker areas in the scene or excessive blooming in the brighter areas of the scene. Blooming is here meant as a localized brightness saturation that spills over to other nearby areas. As a result, intensifier tube imagers and intensified-CCD imaging systems are restricted to relatively small dynamic range imaging; typically no more than about 200 gray levels can be enabled by even the best vacuum tube-based systems, and generally, far fewer gray levels span the restricted intra-scene dynamic range.
Additional inherent limitations of intensifier vacuum tube technology limit the overall performance of intensifier tube imagers and intensified-CCD imaging systems. For example, the finite time required for a phosphor image produced by an intensifier tube to dissipate from the phosphor results in deleterious image artifacts in a temporal sequence of images when there is motion in the scene. In addition, vacuum tubes, being formed of glass, are fragile, and therefore require special handling considerations for the image system in which they are incorporated. The photocathode and micro-channel plate used in the intensifier tube have relatively short life cycles, requiring frequent replacement and repair. Furthermore, vacuum tubes are relatively large in size, limiting the minimum overall imaging system size. Vacuum tubes are also relatively expensive, adding significant cost to the overall cost of the imaging system. Many other performance, handling, and packaging limitations are additionally imposed by the intensifier vacuum tube technology.
Another class of low-light-level imaging systems, known as so-called slow-scan or frame-integrating cameras, do not rely on a gain mechanism to amplify ambient light for producing a viable electronic image. Instead, a slow-scan camera typically includes only a CCD imager that in operation is exposed to a low-light-level scene for an extended period of time, i.e., seconds or longer, during which time the device accumulates a large number of photoelectrons; after a time period sufficient to accumulate an adequate photoelectron count, a viable electronic image can be produced by the CCD camera. This technique overcomes some of the practical limitations of intensifier tube imagers and intensified-CCD image systems, but is inherently limited to extremely slow image capture speeds. Slow-scan cameras thus cannot accommodate real time imaging for production of video sequences at rates even close to about 25-30 frames per second. Indeed, slow-scan cameras are typically used in applications that are not primarily time-sensitive; for example, being used in astronomical applications as an electronic substitute for long-exposure film photography through telescopes.
Many critical low-light-level surveillance and mobility applications require real time digital video imaging of night time scenes, e.g., air-land scenes, characterized by a large intra-scene dynamic range. But like the systems described above, conventional low-light-level imaging systems developed heretofore have provided only suboptimal performance under such complex conditions and are further restricted by additional performance and practical limitations that impede or inhibit a high level of operational performance in applications for which real time low-light-level imaging is critical.